1. Field of the Invention
The present invention relates to a rubber composition for coating a steel cord, a tire reinforcement belt in which the steel cord is coated with the rubber composition, and a pneumatic tire using the tire reinforcement belt.
2. Description of the Background Art
In a pneumatic tire, a tire reinforcement belt including a steel cord and rubber coating the steel cord is often used for the purpose of improving basic characteristics thereof. There are increasing demands on improvement of safety and steering stability of a pneumatic tire, and reduction of rolling resistance. An adhering interface between a steel cord and a rubber of a tire reinforcement belt is liable to be broken due to heat generation and repeated deformation during high-speed running, and such breakage will cause significant damage on the tire. Therefore, it is important to further improve adhesion strength between a steel cord and a rubber.
In general, a steel cord is plated with brass for the purpose of improving reinforcement effect of a belt by increasing adhesion strength with rubber. On the other hand, a rubber composition forming rubber that coats the steel cord sometimes contains a cobalt salt of an organic acid as an adhesion accelerator. When a large quantity of the cobalt salt of an organic acid is used, anti-deterioration adhesiveness is poor and adhesion strength decreases with time, although the adhesion strength directly after vulcanization, namely initial adhesiveness is excellent.
Also known is a technique that blends silica into a rubber composition forming the rubber for the purpose of increasing adhesion strength between rubber and a steel cord. Silica stabilizes the rubber composition by adsorbing moisture by a hydrophilic silanol group on the surface thereof. For example, silica is considered to control deterioration of an adhesion interface by adsorbing moisture present in the rubber composition or atmospheric moisture. However, when silica is blended, Mooney viscosity of the rubber composition increases, so that workability is impaired.
Japanese Patent Laying-Open No. 2005-113015 proposes a technique of containing a nano composite formed by microdispersing layered clay minerals in a rubber component, in a rubber composition for coating a steel cord, thereby improving fatigue resistance of the rubber composition, reducing heat generation, and improving initial adhesion and anti-deterioration adhesiveness of rubber with respect to the steel cord.